Generator protection system

ABSTRACT

A temperature sensor is disclosed. The sensor includes an optical fiber and at least one twin-grating structure formed on the optical fiber. Each twin-grating structure includes a first optical grating structure, a second optical grating structure adjacent the first optical grating structure, and a sensing cavity disposed between the first and second optical grating structures. Each twin-grating structure is selectively responsive to a unique wavelength of light to generate an optical interference fringe signal. For each twin-grating structure, an optical property of the twin-grating structure and a phase of the optical interference fringe signal generated by the twin-grating structure are determined by a temperature of the twin-grating structure.

BACKGROUND

Electric generators, such as those used in the power generationindustry, essentially comprise a rotor and a stator. The rotor is woundwith conductors to form a field winding. The stator is wound withconductors to form a stator winding. The field winding is supplied withan excitation current in order to create a magnetic field on the rotor.When the rotor spins inside the stator, electric power is induced in thestator winding.

In order to ensure safe and efficient operation of an electricgenerator, operating characteristics of the generator may be monitoredusing a number of different instruments located throughout thegenerator. Monitored characteristics may comprise, by way of example,vibration, temperature, voltage and current. Conventionalconductor-based instruments for monitoring such characteristics may beunsuitable due to the harsh operating environment within the generatoror a lack of space necessary to locate such instrumentation on thegenerator component(s) of interest. Instruments that address theselimitations are therefore desirable.

FIGURES

The novel features of the various embodiments are set forth withparticularity in the appended claims. The described embodiments,however, both as to organization and methods of operation, may be bestunderstood by reference to the following description, taken inconjunction with the accompanying drawings in which:

FIG. 1 illustrates a fiber optic flux probe sensor according to oneembodiment;

FIG. 2 illustrates a spectrum of a twin-grating fiber optic sensoraccording to one embodiment;

FIG. 3 illustrates operation of a fiber optic flux probe sensoraccording to one embodiment;

FIG. 4 illustrates a graphical relationship between a phase shift of thefringe signal and an applied magnetic field according to one embodiment;

FIG. 5 illustrates a packaged fiber optic flux probe sensor according toone embodiment;

FIG. 6 illustrates detection of phase shift in fringe signal accordingto one embodiment;

FIG. 7 illustrates a fiber optic flux probe sensor system according toone embodiment;

FIG. 8 illustrates a fiber optic flux probe sensor system for monitoringthe rotor winding insulation according to one embodiment;

FIG. 9 illustrates magnetic flux waveforms detected by a fiber opticflux probe sensor system according to one embodiment;

FIG. 10 illustrates a twin-grating fiber optic sensor according to oneembodiment;

FIG. 11 illustrates a fiber optic wedge tightness sensor array embeddedinside a composite plate according to one embodiment;

FIG. 12 illustrates operation of a fiber optic wedge tightness sensoraccording to one embodiment;

FIG. 13 illustrates a graphical relationship between axial strainapplied on a fiber optic sensor, fringe phase and tightness according toone embodiment;

FIG. 14 illustrates tightness changes in a wedge plate over timeaccording to one embodiment;

FIG. 15 illustrates functions used in a computer user interface programfor wedge tightness monitoring according to one embodiment;

FIG. 16 illustrates a fiber optic wedge tightness sensor systemaccording to one embodiment;

FIG. 17 illustrates a fiber optic wedge tightness sensor system formonitoring tightness variations in multiple wedge plates in the statoraccording to one embodiment;

FIG. 18 illustrates a fiber optic temperature sensor array packagedinside a Teflon tube according to one embodiment;

FIG. 19 illustrates a fiber optic core temperature sensor systemaccording to one embodiment;

FIG. 20 illustrates functions used in a computer user interface programfor core temperature monitoring according to one embodiment;

FIG. 21 illustrates a fiber optic core temperature sensor system formonitoring temperature variations in multiple cores in the statoraccording to one embodiment;

FIG. 22 illustrates a fiber washer with an embedded fiber optic sensoras a fiber optic through bolt sensor according to one embodiment;

FIG. 23 illustrates a fiber optic through bolt sensor array according toone embodiment;

FIG. 24 illustrates a graphical relationship between applied stress on afiber washer and phase shift in a detected fringe signal according toone embodiment;

FIG. 25 illustrates a fiber optic through bolt sensor system accordingto one embodiment;

FIG. 26 illustrates functions used in a computer user interface programfor through bolt tightness monitoring according to one embodiment;

FIG. 27 illustrates a fiber block with fiber optic sensor embedded as apackaged fiber optic parallel ring assembly sensor according to oneembodiment;

FIG. 28 illustrates a fiber optic parallel ring assembly sensor arrayaccording to one embodiment;

FIG. 29 illustrates a fiber optic parallel ring assembly sensor systemaccording to one embodiment;

FIG. 30 illustrates functions used in a computer user interface programfor parallel ring assembly tightness monitoring according to oneembodiment;

FIG. 31 illustrates a packaged fiber optic vibration/temperature sensoraccording to one embodiment;

FIG. 32 illustrates fringe signal movement during reed vibrationaccording to one embodiment;

FIG. 33 illustrates measurement of vibration with a fiber opticvibration/temperature sensor according to one embodiment;

FIG. 34 illustrates movement of a fringe signal with temperatureaccording to one embodiment;

FIG. 35 illustrates graphical relationships between wavelength andtemperature of sensor, between laser wavelength and laser controlcurrents and between laser control currents and the sensor temperatureaccording to various embodiments;

FIG. 36 illustrates a fiber optic vibration/temperature sensor systemaccording to one embodiment;

FIG. 37 illustrates a detection process in a fiber opticvibration/temperature sensor system according to one embodiment;

FIG. 38 illustrates a fiber optic vibration/temperature sensor systemfor monitoring vibration states and temperature variations in main leadsaccording to one embodiment;

FIG. 39 illustrates a fiber optic vibration/temperature sensor systemfor monitoring vibration states and temperature variations in a lead boxaccording to one embodiment;

FIG. 40 illustrates a fiber optic flow sensor assembly according to oneembodiment;

FIG. 41 illustrates packaged fiber optic flow sensors according tovarious embodiments;

FIG. 42 illustrates a fiber optic flow sensor assembly according to oneembodiment;

FIG. 43 illustrates vibration generation in a flow induced by vortexaccording to one embodiment;

FIG. 44 illustrates a graphical relationship between the frequency ofvortex-induced vibration and the flow rate according to one embodiment;

FIG. 45 illustrates vibration generation in a flow induced by a vortexand detection with a plug-in type fiber optic flow sensor according tovarious embodiments;

FIG. 46 illustrates a graphical relationship between a DC level as anoutput of the phase measurement of detected signal and temperaturearound the sensor according to one embodiment;

FIG. 47 illustrates a fiber optic flow sensor system according to oneembodiment;

FIG. 48 illustrates functions used in a computer user interface programfor flow rate and temperature measurements according to one embodiment;

FIG. 49 illustrates a fiber optic flow sensor system for monitoringrestriction of flow with a small rate and increased temperatureaccording to one embodiment;

FIG. 50 illustrates a fiber optic flow sensor system for monitoringrestriction of flow with a large rate and increased temperatureaccording to one embodiment;

FIG. 51 illustrates packaged fiber optic moisture sensors according tovarious embodiments;

FIG. 52 illustrates a graphical relationship between the phase shift ina detected fringe signal and the relative humidity according to oneembodiment;

FIG. 53 illustrates a graphical relationship between the phase shift indetected fringe signal and the temperature around the sensor accordingto one embodiment; and

FIG. 54 illustrates a fiber optic moisture/temperature sensor systemaccording to one embodiment.

DESCRIPTION Fiber Optic Flux Probe Sensor

The winding turn insulation state in a rotor may be an important aspectto be real-time monitored for possible shorted turns arising during theoperation of the rotor. Early information may be helpful when making amaintenance decision concerning when and whether the rotor mightnecessarily be taken out of service and reworked. Previous technologymainly utilizes an air-gap magnetic flux sensor on-line to measure therotor slot leakage flux in the rotor. The transitional magnetic fluxsensor may utilize the Hall probe or the coil as sensing elements whichare electrical in nature. These types of sensors may be connected to thedata acquisition system outside of the generator with an electricalwire. The wire causes problems in the power system security andincreases risk to people when they handle these devices on-line. Asfiber optic sensing technology may provide the sensor system withimmunity to electromagnetic interference, it may be possible to let thefiber optic flux probe sensors work in such a harsh environment. Thefiber optic flux probe sensor proposed may be a fiber-based interferencesensor with ferrite-magnetostrictive coating that may allow fordetection of the fringe signals, induced by the rotor slot leakage fluxin the generator, more efficiently. Due to the direct relationshipbetween rotor winding shorted turns and magnetic flux variation in therotor, the amplitude of variation may be used as an indicator of shortedturns. By incorporating a dual cavity where one cavity is coated withthe magnetostrictive material and the other cavity is longer, giving adifferent fringe pattern, a computer-implemented algorithm may be usedto sort out the effect of the shorted turn as well as the localtemperature rise.

Description of Technology

Magnetic flux leakage inspection methods or tools can be used to locateand characterize the rotor winding where there have been previousepisodes of shorted turns. In principle, as the generator operates,magnetic flux generated by the rotor winding may leak into thesurrounding air or air-gap between the rotor and stator. This leakageflux is known as rotor slot leakage. If a magnetic flux sensor is put inthis leakage region, the sensor may accumulate a physical parameteroutput related to the flux magnitude. The rotor slot leakage may belocal to each rotor slot and its magnitude may be proportional to thecurrent flowing through the turns found in the slot and therefore may bea possible indication of active shorted turns in the slot. Several typesof known sensor systems employing Hall-type or coil-type ormagnetic-resistance-type sensing elements to detect the rotor slot fluxleakages in a generator have been developed. The corresponding softwareenvironment in the computer has been built to automatically determinethe rotor winding shorted turns by analyzing detected flux waveforms.

Embodiments of the fiber optic flux probe sensor may comprise aferrite-coated fringe sensor. The structure of this embodiment is shownin FIG. 1 in which a sensing element with a designed wavelength maycomprise a fringe sensor. The sensor may be coated with amagnetostrictive material, 1.1, such as Terfenol-D. The sensor with twogratings may comprise a Fabry-Perot type of interferometer, 1.2.Therefore it may have very high detection sensitivity and fast responsespeed compared to that of single-grating sensors, 1.3, and therefore maybe very suitable for magnetic flux detections. FIG. 2 is an outputspectrum of the twin-grating sensor, in which many spectral lines,called the interference fringe, can be seen.

In operation principle, as illustrated in FIG. 3, the variation ofapplied magnetic flux through the magnetostrictive effect may affectsome physical parameters of the sensor, such as the working wavelengthand the cavity length, through the photoelasticity of the optical fiber.As a result, it may finally change the phase of the fringe signal. Thephase shift magnitude of the fringe signal may be proportional to theapplied magnetic field in intensity in an effective saturation range asillustrated in FIG. 4.

In one embodiment, Terfenol-D may be selected as a coating material forthe magnetic flux sensor. It has a relatively large magnetostriction onthe order of 1000 ppm for magnetizing a field of 100 mT when it operatesat room temperature, free of mechanical stress. The saturation field ofTerfenol-D depends on the mechanical load and increases from 100 mT to500 mT for loading in the range 0-100 mPa. Additionally, Terfenol-D canoperate efficiently at a frequency range of 0-5 kHz. It may be verysuitable as a magnetostrictive coating material on the fiber sensor forthe detection of rotor slot leakage flux in the generator. For example,where the rotor has 4 poles and 8 coils per pole and the rotor rotatesat 60 Hz rate, the maximum frequency in detection signals generated bythe rotor windings may be about 60 Hz×4×8=1.92 kHz lower than 3 kHz.

One embodiment of a sensor package is schematically illustrated in FIG.5. The fiber flux sensor, 5.1 may be packaged into Teflon, 5.2, withfiberglass filler, 5.3, with 4-mm to 10-mm in thickness for satisfying arequirement of installing in the air-gap of the generator.Mathematically, the phase of fringe signal φ_(FP) can be expressed as:

$\begin{matrix}{{{2{\varphi_{FP}\left( \lambda_{Bragg} \right)}} = \frac{4\pi \; n_{eff}L}{\lambda_{Bragg}}},} & (1)\end{matrix}$

where n_(eff) is an effective refractive index of the optical fiber, Lis the physical length of the sensor cavity and λ_(Bragg) is the centralwavelength of two identical Bragg gratings. When theferrite-magnetostrictive coating, such as Terfenol-D coating is employedon the sensor, the magnetostriction of the coating material may affectthe optical parameters of the sensor in terms of the working wavelengthand effective refractive index as well as the physical length of thecavity. When these optical parameters are changed, the fringe spectrummay move as a blue shift or a red shift in spectral domain and thefringe pattern in time domain, which may present an initial phase changein the fringe time waveform. The magnitude of phase change or phaseshift may be taken as a detectable physical quantity to evaluate themagnitudes of the magnetic flux. When the magnetic flux magnitudechanges with time, the detected fringe may become a time-varied fringesignal.

In one embodiment of a detection process, phase changes in the detectedfringe signal may be measured and then converted into an amplitudevalue, for example, as a voltage value. This signal processing methodmay comprise a fringe tracking algorithm. In operation principle, asshown in FIG. 6, the input detected signal with multiple fringes may befirst filtered in time domain by a time window, 6.1, with a window widthequal to that of the fringe, in order to extract a target fringe, 6.2,to be real-time tracked afterwards. When the laser source is frequencymodulated with a saw-tooth periodic signal, the fringe position in thismodulation signal period (a time that the fringe appears in a modulationperiod) may be determined by the initial phase value of the fringesignal, 6.3. In the detection of the rotor slot leakage, when the rotorrotates, each rotor slot passes over the flux sensor and the slotleakage from that slot may be detected by the fiber optic flux probesensor and converted into a fringe voltage signal as a flux signal, 6.4.

In the shorted-turn sensing algorithm, the premise is that the magnitudeof that peak in the detected flux waveform is related to the amp-turnsin the slot. Since amp-turns are directly related to the number ofactive turns in the slot, it is anticipated that a coil with shortedturns will display a smaller peak than a coil without shorted turns. Bycomparing slot peak magnitudes between poles, the number of shortedturns may be calculated for each coil in the rotor. To calculate thepresence of symmetric shorted turns (same coils in all poles) mayrequire comparison to a base set of data recorded before the developmentof the shorted turns.

A schematic diagram of a fiber optic flux probe sensor system isillustrated in FIG. 7. The system may comprise a fiber optical fluxprobe sensor, 7.1, a data acquisition system, 7.2, and a computer, 7.3.The basic functions of the data acquisition system may comprise opticaldetection of the signal lights from the flux sensor, signal filtering,analog-to-digital conversion and phase measurement.

In the data acquisition system, the fringe signals received may bedigitized with the analog-to-digital converter. The phase information,related to the rotor slot leakage flux, may be extracted by using afringe tracking algorithm as illustrated in FIG. 6. The flux sensingwaveforms, in analog or digital format, may be output or transmitted toa computer for signal processing and waveform analysis. With thesoftware in the computer, a phase mark signal embedded in flux sensingwaveforms may be extracted as a signature to identify the physical poleto which a coil with shorted turns belongs.

It should be noted that the fiber optic flux probe sensor system may notonly provide a basic tool for the rotor winding insulation stateanalysis, but may also simultaneously display the real-time temperaturein the generator. This may be an important factor in determining theoperating status of the rotor when the generator is in a running state.

Example

As an application case, one embodiment of a fiber optic flux probesensor system is schematically illustrated in FIG. 8, in which a fiberoptic flux probe sensor, 8.1, may be mounted on a stator wedge, 8.2, ina position over a continuous ring of wedges. The rotor may have fourpoles, 8.3, and each pole may have eight slots, 8.4. The fiber opticflux probe sensor cable, 8.5, may be routed out of the stator core toconnect a data acquisition system, 8.6, in which the flux waveforms maybe detected, recorded and sent to a computer, 8.7, through USBconnection cable. As an example, two sets of detected flux waveforms arepresented in FIG. 9. In these flux waveforms, the upper trace, 9.1, withhigh amplitude in each flux peak, may be one case without the rotor slotshorted turns occurring, and the lower trace, 9.2, may be one case whereseveral shorted turns occur in slots as indicated by arrows. It will beappreciated that when a slot has a shorted turn, the amp-turn in thisslot may decrease and, as a result, the leakage flux may be reducedaccordingly.

Fiber Optic Wedge Tightness Sensors

When the stator wedge assembly loses its tightness, individual windingsmay become free to move resulting in larger vibration amplitude. Thevibration originates from the electromagnetic field interaction betweenthe rotor and the stator; it is the nature of the machine's normaloperation. Excess vibration may cause rubbing and deteriorate theinsulation layer between windings and eventually cause shorted turns.Tightness can be maintained by inserting a ripple spring between thewedge element and the fiberglass filler that directly presses againstthe stator coil. Tightness of the wedge assembly can be defined as ameasurable physical strain in the fiberglass filler and can help toestimate the magnitude of the pressure exerted by the ripple spring. Asthe ripple spring becomes deformed, it may introduce strain to the wedgeelement underneath. This strain may be measured by embedding a twingratings cavity inside the fiberglass filler. The technology involves afringe-phase measurement algorithm. The vertical pressure generated bythe ripple spring may be transferred into a transverse stress and anaxial strain on the fiber sensor. In this phase measurement method,fringe movements can be detected that are caused by changes in the axialstrain on the fiber sensor which indirectly affects the tightness in thewedge assembly. It may therefore be used as an automated inspection toolfor monitoring the tightness in the stator coil.

The embodiment may comprise sensors with a polyamide coated fiber whichmay be 145 microns, for example, providing excellent adhesion so thepressure from the ripple spring may be entirely transferred into theaxial strain. If an acrylate coated fiber is used, the soft coating mayabsorb some of the strain. Polyamide is a tough polymer that may work ina harsh environment with high temperatures of up to 250° C., and likethe normal fiber, may be immune to high voltages and electromagneticinterference.

Basic detection technology may be based on a measurement of the axialstrain index change on the fiber sensor array. The structure of anindividual twin-grating sensor is schematically illustrated in FIG. 10.The sensor may comprise two identical gratings, 10.1, inscribed in thesingle-mode fiber, 10.2. These two gratings form an optical cavity witha length of L_(core)l, where n_(core) is the index of reflection in thefiber core and l is the physical length of the cavity in the fiber. Whenthe reflection ratios of two gratings are both small, the transferfunction of this twin-grating cavity may work as a two beaminterferometer to generate a fringe signal as the sensor is lit with acoherent laser beam. The fringe signal, formed by the interference ofthe incoming beam and also the first reflected beam, may beam theintensity profile of these fringes and can be expressed as:

$\begin{matrix}{{{I_{cav}(v)} = {2I_{0}\kappa \left\{ {1 + {\cos \left\lbrack {\frac{4\pi \; v}{c}\left( {L + {\Delta \; L}} \right)} \right\rbrack}} \right\}}},{{\Delta \; L} = {{\Delta \; L_{strain}} + {\Delta \; L_{temp}}}}} & (2)\end{matrix}$

where I₀ is the input light intensity, ν is the optical frequency, κ isa receiving sensitivity and κ≦1, and c is the speed of light in avacuum. ΔL is the variation of cavity length, which may comprise twoterms, ΔL_(strain) from the axial strain effect and ΔL_(temp) from thethermal effect on the fiber, respectively.

FIG. 2 is an output spectrum of such a twin-grating sensor, in whichthere are many spectral lines called the interference fringes.

The basic structure of the embodiment to monitor wedge tightness isschematically shown in FIG. 11, in which a sensor array, 11.1, would beembedded inside the fiberglass filler, 11.2, to form a sensing unit inthe stator. In this architecture, one fiber sensor, 11.3, manages oneset of wedges per coil slot in the stator as illustrated in FIG. 11. Inthis way, the tightness state in one wedge element may be detected andidentified through interrogating the corresponding sensor by dataacquisition system.

The detection principle may be explained with schematics as illustratedin FIG. 12. When the wedge assembly is tight, the pressures from theripple spring, 12.1, and the coil, 12.2, below may generate a transversestress in the fiberglass filler, 12.3, which may then transfer into anaxial strain applied on the fiber sensor. This strain may elongate thecavity and the fringes may increase in number as illustrated in FIG.12A.

Over a long period of time, as the material ages, the ripple springgradually loses its strength, 12B. The coil may now vibrate more freelyand cause damage to the insulating layer. There may be a possibility ofgradual reduction of cavity length, 12.4, causing the fringe count to godown. This information may be helpful to the maintenance engineer. Whenthe wedge is tightened, the cavity length may stretch again; restoringto its original state and the fringe number may increase.

The implementation of the system and method may involve the initial setup of measurement references during the installation of the sensorarrays. After the stator coil is tightened by wedging through the ripplespring and fiberglass filler, the generator becomes ready to operateagain. Recording at this time may obtain an initial phase value. Throughcontinuous recordings, the change in tightness may be measured asexplained in following section. The initial phase value may be stored asa reference value to use later for a comparison with an updated phasevalue in another inspection cycle. There may be a linear relationshipbetween the fringe phase and the strain applied onto the fiber sensor,as shown in FIG. 13. As the axial strain on the fiber may be linearlyproportional to the cavity length, the fringe phase becomes a linearfunction of the cavity length. Therefore, a variation in the detectedphase value may be interpreted as a change in the axial strain and thecavity length.

The tightness of the wedge T_(wedge), can be defined as follows:

$\begin{matrix}{{T_{wedge} = {\frac{{\varphi_{current} - \varphi_{static}}}{{\varphi_{initial} - \varphi_{static}}} \times 100\%}},} & (3)\end{matrix}$

where φ_(static), φ_(initial) and φ_(current) are the measured phasevalue of the fringe signal in a static state without any applied straingenerated by external pressure on the fiberglass filler, one after theinstallation of the sensor system as an initial value, and one measuredat routine inspection times, respectively. Generally, the tightnessT_(wedge) in a normal situation, varies gradually with passed time (dayor month or year), decreasing from its initial maximum value ˜100% to asmaller value. When the measured T_(wedge) is lower than a designatedthreshold, an alarm signal may be activated.

Usually, one routine inspection may be carried out in a day cycle or ina month cycle, according to the specific running situation in generator.FIG. 14 is a simulated curve of the tightness change in one wedge, whichmay be obtained by following the detection procedures shown in theflowchart of FIG. 15. With this trending plot of FIG. 14, one may learnabout the variation in tightness in the wedge assembly over time.

Phase Measurement Technology

During the measurement process, the phase changes in the fringe can bemeasured and converted into an analog value, for example, as voltages orcurrent. In operation principle, as shown in FIG. 6, the input detectedsignal with multiple fringes may be first filtered in time domain by atime window, 6.1, with a window width equal to that of the fringe, inorder to extract a target fringe, 6.2, to be real-time trackedafterwards. When the laser source is frequency modulated with asaw-tooth periodic signal, the fringe position in this modulation signalperiod (a time that the fringe appears in a modulation period) may bedetermined by the initial phase value of the fringe signal, 6.3. Withthis phase tracking algorithm, very tiny changes in the axial strain maybe caught, 6.4. For a large change in the axial strain, a fringecounting method, to measure the phase changed with the strain, may alsobe used.

Data Acquisition System

A schematic diagram of the fiber optic wedge tightness sensor system isshown in FIG. 16. The basic system (single channel) may comprise a fibersensor array that may be embedded inside fiberglass filler, 16.1, and adata acquisition system. For multiple channels sensing, the dataacquisition system may be able to handle multiple fiber sensor arrays toform a sensor network, 16.2, to manage multiple slots and wedges.

In order to interrogate multiple fiber sensors in different positionsfor monitoring multiple points in the wedge assembly, the dataacquisition system, 16.3, may be required to be capable of working in awavelength-selectable or wavelength division multiplexing mode (WDM) tosweep over the working wavelength of the laser source repeatedly. Thismay be completed by using a wavelength tunable laser diode, 16.4, in thesensor system. Additionally, since there are so many wedge slots in onegenerator system to be inspected, the wavelength tunable range may berequired to be as wide as possible in order to manage as many sensors aspossible in a single fiber. The data acquisition system may also berequired to have multiple-channel detection ability, 16.5, which mayallow each channel to share one laser source and individuallyinterrogate the sensor with a wavelength matching with that of lasersource.

In order to complete a measurement of the phase variation in the fringe,the optical frequency of the laser diode may be swept by changing itsdriving current with a linear modulation waveform, for example, with asaw-tooth waveform signal. This is a frequency division multiplexingworking mode (FDM). Therefore one embodiment of a data acquisitionsystem for wedge tightness monitoring may be able to alternately workunder two different working modes, WDM and FDM, 16.6, as illustrated inFIG. 16. Finally, the detected phase value data from different fibersensors, after preliminarily processing, may be transmitted in digitalform to a computer as a test data server where additional signalprocessing may be performed.

A diagram for illustrating data processing functions in a computer userinterface program is shown in FIG. 15. As illustrated, this software maybe able to make adjustment to the strain measurement. When there is atightness alert, compensation for temperature change may be taken intoconsideration to alleviate concerns of a false alarm.

Example

One embodiment of a fiber optic wedge tightness sensor system isschematically shown in FIG. 17, in which a data acquisition system maysimultaneously handle 16 channels, 17.1, for signal detections. Eachfiber sensor array may be embedded inside a long, fiberglass filler,17.2. The array may comprise up to 70 sensors with different wavelengthsacross the ITU grid in the C-band (1530 nm-1565 nm) A total number of1120 (16×70) fiber sensors are capable of being handled by this dataacquisition system which, in the worst case, would still satisfy therequirement of a large generator.

The detected phase data finally may be transmitted into the computerwith the user interface software, 17.3, as introduced above, where finaldata processing for each sensor may be performed. The calculatedtightness as detection data may be recorded and stored as an inspectionrecord or working report, according to the user's requirement.

Fiber Optic Core Temperature Monitoring

When a short occurs within the interlaminar insulation system in thestator core, extra heat may be generated which may cause the temperatureof the core to increase rapidly. Therefore, the temperature changes inthe core of the stator may be monitored real-time and an alarm may beset off when the temperature in a core increases above a threshold. FromEquation (2), it will be appreciated that the cavity length of thesensor may be a function of both the strain and temperature, so the samedetection principle used in monitoring the wedge tightness may also beemployed to monitor the temperature of the core. As shown in FIG. 18,the similar twin-grating sensor array, 18.1, may be shielded in asmall-size tube, 18.2 (e.g., a Teflon tube) and may be placed in directcontact with the core without any additional pressure. In this way thesensor may be, materially, free of strain and may rapidly detect thetemperature change in the core. Each sensor, 18.3, may manage a sectionof the core and may be registered in the data acquisition system. Aslightly modified detection algorithm, originally used for monitoringthe wedge tightness, may be employed to monitor the temperature of thecore. The movement of the fringe signal in a sensor may be consideredfrom a temperature change in the corresponding core.

Both sensor and the fiber may be polyamide coated in order to providethe sensor an ability to work in a harsh environment with hightemperatures of up to 250° C. Also, just like the normal fiber, thepolyamide coated fiber sensor may be immune to high voltages andelectromagnetic interference.

The phase measurement technology used for temperature detection of thecore may be similar to the monitoring of the wedge tightness. Inoperation principle, as shown in FIG. 6, the input detected signal withmultiple fringes may be first filtered in time domain by a time window,6.1, with a window width equal to that of the fringe, in order toextract a target fringe, 6.2, to be real-time tracked afterwards. Whenthe laser source is frequency modulated with a saw-tooth periodicsignal, the fringe position in this modulation signal period (a timethat the fringe appears in a modulation period) may be determined by theinitial phase value of the fringe signal, 6.3. With this phase trackingalgorithm, very tiny changes in the core temperature may be caught. Fora large change in the core temperature, a fringe counting method may beused to decide the degree of temperature changes in the core.

A schematic diagram of fiber optic core temperature monitoring system isshown in FIG. 19. The basic system (single channel) may comprise a fibersensor array, 19.1, and a data acquisition system. For multiple channelssensing, the data acquisition system may be able to handle multiplefiber sensor arrays to form a sensor network, 19.2, to manage multiplecore temperatures.

In order to interrogate multiple fiber sensors in different positionsfor monitoring the temperature change of each section of correspondingcore, the data acquisition system may be required to be capable ofworking in a wavelength-selectable or wavelength division multiplexingmode (WDM), 19.3, to sweep over the working wavelength of the lasersource repeatedly. This may be completed by using a wavelength tunablelaser diode, 19.4, in the sensor system. Additionally, since there aremultiple sections in the stator core to be inspected, the wavelengthtunable range may be required to be as wide as possible in order tomanage as many sensors as possible in a single fiber. The dataacquisition system may also be required to have a multiple-channeldetection ability, 19.5, which may allow each channel to share one lasersource and individually interrogate the sensor with a wavelengthmatching that of the laser source.

In order to complete a measurement of the phase variation in the fringe,the optical frequency of the laser diode may be swept by changing itsdriving current with a linear modulation waveform, for example, with asaw-tooth waveform signal. This is a frequency division multiplexingworking mode (FDM). Therefore one embodiment of a data acquisitionsystem for core temperature monitoring may be able to alternately workunder two different working modes, WDM and FDM, 19.6, as illustrated inFIG. 19. Finally, the detected phase value data from different fibersensors, after preliminarily processing, may be transmitted in a digitalform to a computer where additional signal processing may be performed.

A diagram for illustrating data processing functions in a computer userinterface program is shown in FIG. 20. As illustrated, this software maydisplay the temperature and change trend of each core to be monitoredand set off an alarm if the temperature is higher than a threshold setpreviously by the user.

Example

One embodiment of a fiber optic core temperature monitoring system isschematically shown in FIG. 21, in which the data acquisition system maysimultaneously handle 16 channels, 21.1, for core temperaturemonitoring. Each sensor array may be placed below the composite fillers,21.2, in contact with the core. One sensor array may comprise up to 70sensors with different wavelengths across the ITU grid in the C-band(1530 nm-1565 nm). A total number of 1120 (16×70) fiber sensors arecapable of being handled by this data acquisition system which, in theworst case, would still satisfy the requirement of a large generator.

The detected phase data finally are transmitted into the computer withthe user interface software, 21.3, as introduced above, where final dataprocessing and temperature calculation for each sensor may be performed.

Fiber Optic Through Bolt Sensors

The bolt tightness in a generator may be real-time monitored throughcertain detection methods. An effective detection method may be to usefiber sensor technology to measure the tightness changes according tothe changes of the optical signal parameters, such as optical intensity,polarization state or optical phase. This embodiment may comprise afiberglass washer, with an embedded fiber optic sensor as a sensingelement, to be used as a through bolt sensor to detect the relativetorque applied on the washer by the bolt. The phase change in theoptical detection signal may be used as a characteristic to determinethe state of the bolt tightness.

A schematic view of the fiber optic through bolt sensor is shown in FIG.22, in which a twin-grating fiber sensor, 22.1, as a sensing elementwith a designed wavelength, may be embedded into the fiberglass washer.The structure of an individual twin-grating sensor is schematicallyillustrated in FIG. 10. The fiber optic through bolt sensor may have twofiber ports (22.2, 22.3), which, as pictured in FIG. 23, may be utilizedto connect to the data acquisition system, 23.1, or cascade with anothersensor by splicing, 23.2, to form a fiber optic through bolt sensorarray, 23.3, that may monitor multiple bolts' tightness in one detectionperiod. In a sensor array, each fiber optic through bolt sensor may havea unique wavelength and the information generated by this sensor may beinterrogated by a data acquisition system operated at a wavelengthmatched with that of the sensor. For interrogating multiple sensors, awavelength-tunable laser source in the data acquisition system may berequired.

A stress on the washer generated by tightening the bolt may generate achange of effective refractive index of the fiber in the fiberglasswasher. This change in turn may alter the optical parameters of thetwin-grating cavity, such as the cavity length as described withEquation (2) on page 10, which may make the phase of interference signal(fringe) change. This phase shift may be linearly proportional to theamount of stress or torque applied on the washer, as illustratedschematically in FIG. 24. The total phase shift, as a result of themoving fringe, may be measured with the phase algorithm technology.

The phase algorithm technology used for bolt tightness monitoring may bethe same as in monitoring the wedge tightness. In operation principle,as shown in FIG. 6, the input detected signal with multiple fringes maybe first filtered in time domain by a time window, 6.1, with a windowwidth equal to that of the fringe, in order to extract a target fringe,6.2, to be real-time tracked afterwards. When the laser source isfrequency modulated with a saw-tooth periodic signal, the fringeposition in this modulation signal period (a time that the fringeappears in a modulation period) may be determined by the initial phasevalue of the fringe signal, 6.3. With this phase tracking algorithm,changes in the fringe signal may be detected. For relatively largechanges in the bolt tightness, a fringe counting method may be used todecide the amount of phase changes in the fringe signal.

Using this phase algorithm technology, as the state of bolt tightnesschanges, a phase variation in the fringe signal may be detected, and thechange trend of the bolt tightness may also be obtained by comparing thecurrently measured value with the previously measured value.

Example

One embodiment of a fiber optic through bolt sensor system to monitorthe tightening states of multiple through bolts in the generator isshown in FIG. 25, in which a wavelength-tunable laser source, 25.1, maybe used. The data acquisition system may simultaneously handle Nchannels, 25.2, and each channel may comprise m through bolt sensors,25.3, which may form a sensor network, 25.4. The wavelength of laserlight may be changed periodically from)λ₁ to λ_(m), 25.5. In this way,each sensor with a designed wavelength may be accessed by the dataacquisition system. The laser driving current may be modulated with asaw-tooth signal; in this way the fringes in the optical cavity of thesensor may be read out. The detected fringe signals in analog format maybe converted into those in digital format via an analogue-to-digitalconverter (ADC) and then may be fed into a micro-processing unit (MPU)for preliminary data processing. The processed data then may betransmitted to a computer via a USB cable. In the computer, as describedin functional blocks in FIG. 26, the received data may be processed tocompensate for the temperature effects. The bolt tightness change trendmay be calculated and the two data sets compared: the current one andthe previous one. If the bolt tightness decreases lower than thedesignated threshold an alarm may be raised.

Fiber Optic Parallel Ring Assemblies Sensors

The block tightness in a generator may be real-time monitored throughcertain detection methods. An effective detection method may be to usefiber sensor technology to measure the tightness changes according tothe changes of the optical signal parameters, such as optical intensity,polarization state or optical phase. In this embodiment, a fiberglassblock, with an embedded fiber optic sensor as a sensing element, may beused as a parallel ring assembly sensor to detect the pressure, or lackthereof, applied on the block by the bolt. The phase change in theoptical detection signal may be used as a characteristic to determinethe state of the block tightness.

A schematic of the fiber optic parallel ring assemblies sensor is shownin FIG. 27, in which a twin-grating fiber sensor, 27.1, as a sensingelement with a designed wavelength, may be embedded into fiberglassblock, 27.2. The structure of a proposed individual twin-grating sensoris schematically illustrated in FIG. 10. The fiber optic parallel ringassemblies sensor may have two fiber ports (27.3, 27.4), which, aspictured in FIG. 28, may be utilized to connect to the data acquisitionsystem, 28.1, or cascade with another sensor by splicing, 28.2, to forma fiber optic parallel ring assemblies sensor array, 28.3, that may beused to monitor multiple block tightness in one detection period. In asensor array, each fiber optic parallel ring assembly sensor may have aunique wavelength and the information generated by this sensor may beinterrogated by a data acquisition system operated at a wavelengthmatched with that of sensor. For interrogating multiple sensors, awavelength-tunable laser source in the data acquisition system may berequired.

Stress on the block, generated by tightening the bolt, may generate achange of effective refractive index of the fiber in the fiberglassblock. This change may in turn alter the optical parameters of thetwin-grating cavity, such as the cavity length as described withEquation (2), which may make the phase of interference signal (fringe)change. This phase shift may be linearly proportional to the amount ofstress or torque applied, as illustrated schematically in FIG. 24. Thephase shift, as fringe movement, may be completed with the phasemeasurement technology.

The phase measurement technology used for block tightness monitoring maybe the same as that used for monitoring the wedge tightness. Inoperation principle, as shown in FIG. 6, the input detected signal withmultiple fringes may be first filtered in time domain by a time window,6.1, with a window width equal to that of the fringe, in order toextract a target fringe, 6.2, to be real-time tracked afterwards. Whenthe laser source is frequency modulated with a saw-tooth periodicsignal, the fringe position in this modulation signal period (a timethat the fringe appears in a modulation period) may be determined by theinitial phase value of the fringe signal, 6.3. With this phase trackingalgorithm, changes in the fringe signal may be detected. For arelatively large change in the block tightness, a fringe counting methodmay be used to determine the amount of phase changes in the fringesignal.

Using this phase measurement technology, as the state of block tightnesschanges, a phase variation in the fringe signal may be detected, and thechange trend of the block tightness may also be obtained by comparingthe currently measured value with the previously measured value.

Example

One embodiment of a fiber optic parallel ring assembly sensor system tomonitor the tightening states of multiple blocks in the generator isshown in FIG. 29, in which a wavelength-tunable laser source, 29.1, maybe used. The data acquisition system may simultaneously handle Nchannels, 29.2, and each channel may comprise m parallel ring assemblysensors, 29.3, which may form a sensor network, 29.4. The wavelength oflaser light may be changed periodically from λ₁ to λ_(m), 29.5. In thisway, each sensor with a designed wavelength may be accessed by the dataacquisition system. The laser driving current may be modulated with asaw-tooth signal; in this way the fringes in the optical cavity of thesensor may be read out. The detected fringe signals in analog format areconverted into those in digital format via an analogue-to-digitalconverter (ADC) and then are fed into a micro-processing unit (MPU) forpreliminary data processing. The processed data then are transmitted toa computer via a USB cable. In the computer, as described in functionalblocks in FIG. 26, the received data may be processed to compensate forthe temperature effects. The block tightness change trend may becalculated and the two data sets compared: the current one and theprevious one. If the block tightness decreases lower than a threshold analarm may be raised.

Fiber Optic Parallel Ring Temperature Sensors

When the connection in the parallel ring is in poor contact, extra heatmay be generated owing to an increase of the contact resistance, whichmay cause the temperature of the parallel ring to increase rapidly.Therefore temperature rises in the parallel rings of the stator may bemonitored real-time and an alarm may be activated when the temperaturein a parallel ring increases above a designated threshold. From Equation(2), it will be appreciated that the cavity length of the sensor may bea function of both the strain and temperature, so the same detectionprinciple used in monitoring the block tightness may also be employed tomonitor the temperature of the parallel ring. As shown in FIG. 18, atwin-grating sensor array may be shielded in a small-size tube (e.g., aTeflon tube) and may be placed in direct contact with parallel ringswithout any additional pressure. In this way the sensor may be free ofstrain and may rapidly detect a temperature rise in the parallel ringmonitored. Each sensor may manage a part of the parallel rings and maybe registered in the data acquisition system. A slightly modifieddetection algorithm, originally used for monitoring the block tightness,may be employed to monitor the temperature rise of the parallel ring.The movement of the fringe signal in a sensor may be considered from atemperature rise in the corresponding monitoring area.

Both sensor and the fiber are polyamide coated which may provide thesensor an ability to work in harsh environments with high temperaturesof up to 250° C. Also, just like the normal fiber, the polyamide coatedfiber sensor may be immune to high voltages and electromagneticinterference.

The phase measurement technology used for temperature detection of theparallel ring may be the same as in monitoring block tightness. Inoperation principle, as shown in FIG. 6, the input detected signal withmultiple fringes may be first filtered in time domain by a time window,6.1, with a window width equal to that of the fringe, in order toextract a target fringe, 6.2, to be real-time tracked afterwards. Whenthe laser source is frequency modulated with a saw-tooth periodicsignal, the fringe position in this modulation signal period (a timethat the fringe appears in a modulation period) may be determined by theinitial phase value of the fringe signal, 6.3. With this phase trackingalgorithm, changes in the ring temperature may be detected. Forrelatively large temperature changes, a fringe counting method may beused to determine the degree of temperature rises in the parallel ring.

A schematic diagram of fiber optic parallel ring temperature monitoringsystem is shown in FIG. 19. The basic system (single channel) maycomprise a fiber sensor array and a data acquisition system. Formultiple channels sensing, the data acquisition system may be able tohandle multiple fiber sensor arrays, 19.1, to form a sensor network,19.2, to manage multiple parallel ring temperatures.

In order to interrogate multiple fiber sensors in different positionsfor monitoring the temperature rise of corresponding parallel rings, thedata acquisition system may be required to be capable of working in awavelength-selectable or wavelength division multiplexing mode (WDM),19.3, to sweep over the working wavelength of the laser sourcerepeatedly. This may be completed by using a wavelength tunable laserdiode, 19.4, in the sensor system. Additionally, since there are so manyparallel rings in stator to be inspected, the wavelength tunable rangemay be required to be as wide as possible in order to be able to manageas many sensors as possible in a single fiber. The data acquisitionsystem may also be required to have a multiple-channel detection abilitywhich allows each channel to share one laser source and individuallyinterrogate the sensor with a wavelength matching that of the lasersource.

In order to complete a measurement of the phase variation in the fringe,the optical frequency of the laser diode may be swept by changing itsdriving current with a linear modulation waveform, for example, with asaw-tooth waveform signal. This is a frequency division multiplexingworking mode (FDM). Therefore one embodiment of a data acquisitionsystem may be able to alternately work under two different workingmodes, WDM and FDM, 19.6. Finally, the detected phase value data fromdifferent fiber sensors, after preliminarily processing, may betransmitted in digital form to a computer where additional signalprocessing may be performed.

A diagram for illustrating data processing functions in a computer userinterface program is shown in FIG. 30. As illustrated, this software maybe able to display the temperature and change trend of each parallelring to be monitored and set off an alarm if the temperature is higherthan a threshold set previously by the user.

Fiber Optic Coil Connection Temperature Sensors

When the connection in the coil is in poor contact, extra heat may begenerated owing to an increase of the contact resistance, which maycause the temperature of the coil connection to increase rapidly.Therefore the temperature rise in the coil connection may be monitoredreal-time and an alarm may activate when the temperature in a coilconnection increases over a designated threshold. From Equation (2), itwill be appreciated that the cavity length of the sensor may be afunction of both the strain and temperature, so the same detectionprinciple used in monitoring block tightness may also be employed tomonitor the temperature rise in the coil connection. As shown in FIG.18, the similar twin-grating sensor array may be shielded in asmall-size Teflon tube and placed in direct contact with coils withoutany additional pressure. In this way, the sensor may be free of strainand may rapidly detect the temperature rise in the coil connectionmonitored. Each sensor may manage a coil connection and may beregistered in the data acquisition system. A slightly modified detectionalgorithm originally used for monitoring the coil tightness may beemployed to monitor the temperature rise of the coil connection. Themovement of the fringe signal in a sensor may be considered from atemperature rise in the corresponding coil connection to be monitored.

Both the sensor and the fiber may be polyamide coated which may providethe sensor an ability to work in a harsh environment with hightemperatures of up to 250° C. Also just like the normal fiber, thepolyamide coated fiber sensor may be immune to high voltages andelectromagnetic interference.

The phase measurement technology used for temperature detection of thecoil connection may be the same as used for monitoring block tightness.In operation principle, as shown in FIG. 6, the input detected signalwith multiple fringes may be first filtered in time domain by a timewindow, 6.1, with a window width equal to that of the fringe, in orderto extract a target fringe, 6.2, to be real-time tracked afterwards.When the laser source is frequency modulated with a saw-tooth periodicsignal, the fringe position in this modulation signal period (a timethat the fringe appears in a modulation period) may be determined by theinitial phase value of the fringe signal, 6.3. With this phase trackingalgorithm, a temperature rise in a coil connection may be detected. Fora large temperature change, a fringe counting method may be used todetermine the degree of temperature rises in a coil connection.

A schematic diagram of fiber optic coil connection temperaturemonitoring system is shown in FIG. 19. The basic system (single channel)may comprise a fiber sensor array and a data acquisition system. Formultiple channels sensing, the data acquisition system may be able tohandle multiple fiber sensor arrays, 19.1, to form a sensor network,19.2, to manage multiple coils.

In order to interrogate multiple fiber sensors in different positionsfor monitoring the temperature rise of corresponding coil connection,the data acquisition system may be required to be capable of working ina wavelength-selectable or wavelength division multiplexing mode (WDM),19.3, to sweep over the working wavelength of the laser sourcerepeatedly. This may be completed by using a wavelength tunable laserdiode, 19.4, in the sensor system. Additionally, since there are so manycoils in the generator to be inspected, the wavelength tunable range maybe required to be as wide as possible in order to be able to manage asmany sensors as possible in a single fiber. The data acquisition systemmay also be required to have a multiple-channel detection ability, 19.5,which may allow each channel to share one laser source and individuallyinterrogate the sensor with a wavelength matching with that of lasersource.

In order to complete a measurement of the phase variation in the fringe,the optical frequency of the laser diode may be swept by changing itsdriving current with a linear modulation waveform, for example, with asaw-tooth waveform signal. This is a frequency division multiplexingworking mode (FDM). Therefore one embodiment of a data acquisitionsystem may be able to alternately work under two different workingmodes, WDM and FDM, 19.6. Finally, the detected phase value data fromdifferent fiber sensors, after preliminarily processing, may betransmitted in digital form to a computer where additional signalprocessing may be performed.

A diagram for illustrating data processing functions in a computer userinterface program is shown in FIG. 30. As illustrated, this software maybe able to display the temperature and change trend of each coilconnection monitored and set off an alarm if the temperature is higherthan a threshold set previously by the user.

Fiber Optic Main Lead Vibration/Temperature Sensors

The vibration and temperature of the main lead in a generator are twoimportant parameters which may be real-time monitored through certaindetection methods. An increase in the vibration amplitude of the mainlead indicates that the main lead may have lost its tightness andintegration, and may become free to move. As a result, excess vibrationmay cause a failure and eventually may cause shorted turns. Accompanyingthis process, there may be excess heat generated which may raise thetemperature of the main lead. An effective detection method may be touse fiber sensor technology to measure the changes of the optical signalparameters, such as optical intensity, polarization state or opticalphase, induced by these effects. In this embodiment, a packaged fiberoptic vibration/temperature sensor may be used to simultaneously monitorthe vibration of the main lead and the temperature changes around thesensor. The movements of the detected fringe signal in high speed and inlow speed, for example, may be used as two characteristics to determinethe vibration state of the main lead and the temperature variations,respectively.

Description of Technology

As shown in FIG. 31, a vibration/temperature sensor may comprise atwin-grating fiber sensor, 31.1, as a sensing element, a fiberglass reedforming a vibration board, 31.2, and a nylon box with fiberglassfilling, 31.3. The sensing element may be embedded in the fiberglassreed which may be mounted on a pedestal, 31.4, in the sensor box forminga mechanical amplifier.

The sensor's cavity may be formed by two identical fiber gratings (twingratings), 31.5, in respect to the Bragg wavelength and the reflectionrate. In the manufacturing process, the twin gratings may be inscribedin the fiber within a single exposure session by the use of amplitudemasks with two equal slits separated by a designed distance. Thisprocess theoretically guarantees two FBGs be formed with identicaloptical characteristics. The sensor finally may be packaged in a sealedbox to prevent the ingress of dust and oil or other contaminants. Asillustrated schematically by FIG. 32, when the sensor is mounted on anobject with mechanical vibration, the reed may vibrate and deform, 32.1,periodically owing to an acceleration applied on the reed, 32.2, whichmay generate a time-varied stress on the optical cavity structure of thesensor and which may, in turn, induce a phase periodic change, regardedas fringe movement, 32.3, in the detected fringe signal.

FIG. 33 is a schematic for the vibration measurement mechanism whichinvolves an algorithm that tunes the laser central wavelength to themid-point of a fringe signal through adjusting the driving current ofthe laser indicated with a letter M, 33.1. This may be regarded as anintersection point or a working point generally used for phasemeasurements in the sensor technology. When the fringe signal moves toleft and right, 33.2, periodically as a result of the vibration of thereed, the working point of the detection system may change, 33.3, alongwith the slope of the target fringe, moving to the upper portion (Hpoint) and the lower portion (L point) periodically, which may give adifferent intensity of the reflected light. This may form an effectivetranslation mechanism to convert a wavelength-changed signal to anintensity-changed signal. In this way, the intensity changes in thedetection signal may then be interpreted as a detected vibration signalin the analog form.

Detection of temperature changes during vibration measurements may becompleted by adjusting the laser central wavelength to track the targetfringe movement. As shown in FIG. 34, when the temperature around thesensor changes, for example from T₁, to T₂, the wavelength of the fringeto be detected may shift accordingly from λ_(C) to λ_(D). This processmay induce the fringe to move toward the long wavelength which, as aresult of working point changing, may induce a decrease of the outputamplitude of the vibration signal. In order to keep the output amplitudeof the vibration signal constant, the laser central wavelength may betuned to dynamically track the target fringe movement. As illustrated inFIG. 34, the laser central wavelength may be adjusted from λ_(P) toλ_(Q), which, theoretically, makes the working point recover to theoriginal position and consequently keeps the vibration signal amplitudeconstant. This fringe-tracking process may be completed by adjusting thelaser drive current based on an error signal measured from the deviationof the vibration signal amplitude.

While adjusting the drive current of the laser diode may be fast enoughto tune the laser central wavelength to track the target fringe movementto readjust the working point, the technique may have a limited tuningrange. Beyond this range, the laser central wavelength may be tuned withthe help of the thermoelectric cooler (TEC) drive current adjustment. Asknown, the laser central wavelength changes linearly with the operationtemperature that may be controlled by the TEC and may have a largetuning span and change rate per ° C. Therefore, besides the laser drivecurrent control, the TEC's drive current control may be used to tune thelaser central wavelength to track the target fringe movement within alarge temperature span. Basically, the laser's drive current control maybe employed for fine wavelength tuning, while the TEC's drive currentcontrol may be employed for wavelength tuning in a larger scale. The twocontrol methods may be alternately employed in the sensor system forachieving an accurate adjustment of the laser center wavelength to trackthe target fringe movement. Based on this control algorithm, thevariations of the vibration signal amplitude from the working pointmovement, due to the temperature changes, may be compensated. At thesame time, the deviation values in TEC's drive current and the laserdrive current may be read out as a measured value for estimating thetemperature changes affecting the sensor head. The relationships betweenthe sensor temperature, which results in sensor wavelength shift, maythen be compensated by the laser wavelength shift due to changes in thedrive currents of the laser and the TEC, as shown in FIGS. 35(1), (2)and (3), respectively. From these relationships, it will be appreciatedthat the temperature change around the sensor may be obtained from thedeviation values of drive currents of the laser and TEC.

Example

One embodiment of a sensor system may be able to operate in amulti-channel mode by using multiplexing technology to monitor thevibration and the temperature variation at different main leadssimultaneously, as shown in FIG. 36, where N sensors may be employed forsimultaneously monitoring N-point vibrations and the temperaturevariations in the generator. The reflected lights from each sensor maybe detected, 36.1 and the vibrations and temperature variations at eachmonitoring point may be read out and calculated, 36.2, respectively,with the algorithm mentioned above. The detailed process for possiblevibration and temperature variation detections for each sensor isexplained with a flowchart as shown in FIG. 37. In one embodiment, asshown in FIG. 38, a sensor system may use a distributed feedback (DFB)laser as the light source and six sensors, 38.1, to monitor thevibrations and temperature variations at six different main leads in thegenerator. To measure temperature changes while the vibration ismeasured, the laser drive current may be adjusted one step lower than 1mA, so that the intensity of the laser output is not affectedexcessively. For example, the system may calculate the temperaturechange as an indirect temperature measurement around the sensor bymultiplying the total changed current values with the followingparameters: for each ° C. change in laser chip temperature, the centralwavelength of the laser may move by 100 picometer; and for the drivingcurrent change of every 1 mA, the central wavelength of the laser mayshift by 10 picometer. This control mechanism may balance the sensorwavelength shift in a change rate up to 10 picometer per ° C. Themeasured data may be transmitted from the data acquisition system, 38.2,to a computer, 38.3, via a USB cable. In the computer, a graphical userinterface program may show the detected multi-channel vibrationwaveforms, signal spectrums and temperature change trends. Using theuser interface program, the user could set the alarm threshold for eachmonitoring point in the main leads.

Fiber Optic End Winding Vibration/Temperature Sensors

The vibration and temperature of the end winding in a generator are twoimportant parameters which may be real-time monitored through certaindetection methods. An increase in the vibration amplitude of the endwinding may indicate that the end winding has lost its tightness andintegration, and become free to move. As a result, excess vibration maycause failure of the windings. Accompanying this process, there may beexcess heat generated which may raise the temperature of the winding. Aneffective detection method may be to use fiber sensor technology tomeasure the changes of the optical signal parameters, such as opticalintensity, polarization state, or optical phase that may be induced bythese effects. In this embodiment, a packaged fiber opticvibration/temperature sensor may be used to simultaneously monitor thevibration of the end winding and the temperature changes around thesensor. The movements of the detected fringe signal in high speed and inlow speed may be used as two characteristics to determine the vibrationstate of the end winding and the temperature variations, respectively.

Description of Technology

A vibration/temperature sensor, as shown in FIG. 31, may comprise atwin-grating fiber sensor, 31.1, as a sensing element, a fiberglass reedforming a vibration board, 31.2, and a nylon box with fiberglassfilling, 31.3. The sensing element may be embedded in the fiberglassreed which may be mounted on the pedestal, 31.4, in the sensor boxforming a mechanical amplifier.

The sensor's cavity may be formed by two identical fiber gratings (twingratings), 31.5, in respect to the Bragg wavelength and the reflectionrate. In the manufacturing process, the twin gratings may be inscribedin the fiber within a single exposure session by the use of amplitudemasks with two equal slits separated by a designed distance. Thisprocess theoretically guarantees two FBGs be formed with identicaloptical characteristics. The sensor finally may be packaged in a sealedbox to prevent the ingress of dust and oil.

As illustrated schematically by FIG. 32, when the sensor is mounted onan object with mechanical vibration, the reed may vibrate and deform,32.1 periodically owing to an acceleration applied on the reed, 32.2,which may generate a time-varied stress on the optical cavity structureof the sensor and which may, in turn, induce a phase periodic change,regarded as fringe movement, 32.3, in the detected fringe signal.

FIG. 33 is a schematic for the vibration measurement mechanism whichinvolves an algorithm that tunes the laser central wavelength to themid-point of a fringe signal through adjusting the driving current ofthe laser indicated with a letter M, 33.1. This may be regarded as anintersection point or a working point generally used for phasemeasurements in the sensor technology. When the fringe signal moves toleft and right, 33.2, periodically as a result of the vibration of thereed, the working point of the detection system may change, 33.3, alongwith the slope of the target fringe, moving to the upper portion (Hpoint) and the lower portion (L point) periodically, which may give adifferent intensity of the reflected light. This may form an effectivetranslation mechanism to convert a wavelength-changed signal to anintensity-changed signal. In this way, the intensity changes in thedetection signal may then be interpreted as a detected vibration signalin the analog form.

Detection of temperature changes during vibration measurements may becompleted by adjusting the laser central wavelength to track the targetfringe movement. As shown in FIG. 34, when the temperature around thesensor changes, for example from T₁, to T₂, the wavelength of the fringeto be detected may shift accordingly from λ_(C) to λ_(D). This processmay induce the fringe to move toward the long wavelength which, as aresult of working point changing, may induce a decrease of the outputamplitude of the vibration signal. In order to keep the output amplitudeof the vibration signal constant, the laser central wavelength may betuned to dynamically track the target fringe movement. As illustrated inFIG. 34, the laser central wavelength may be adjusted from λ_(P) toλ_(Q), which, theoretically, makes the working point recover to theoriginal position and consequently keeps the vibration signal amplitudeconstant. This fringe-tracking process may be completed by adjusting thelaser drive current based on an error signal measured from the deviationof the vibration signal amplitude.

While adjusting the drive current of the laser diode may be fast enoughto tune the laser central wavelength to track the target fringe movementto readjust the working point, the technique may have a limited tuningrange. Beyond this range, the laser central wavelength may be tuned withthe help of the thermoelectric cooler (TEC) drive current adjustment. Asknown, the laser central wavelength changes linearly with the operationtemperature that may be controlled by the TEC and may have a largetuning span and change rate per ° C. Therefore, besides the laser drivecurrent control, the TEC's drive current control may be used to tune thelaser central wavelength to track the target fringe movement within alarge temperature span. Basically, the laser's drive current control maybe employed for fine wavelength tuning, while the TEC's drive currentcontrol may be employed for wavelength tuning in a larger scale. The twocontrol methods may be alternately employed in the sensor system forachieving an accurate adjustment of the laser center wavelength to trackthe target fringe movement. Based on this control algorithm, thevariations of the vibration signal amplitude from the working pointmovement, due to the temperature changes, may be compensated. At sametime, the deviation values in TEC's drive current and the laser drivecurrent may be read out as a measured value for estimating thetemperature changes affecting the sensor head. The relationships betweenthe sensor temperature, which results in sensor wavelength shift, maythen be compensated by the laser wavelength shift due to changes in thedrive currents of the laser and the TEC, as shown in FIGS. 35(1), (2)and (3), respectively. From these relationships, it will be appreciatedthat the temperature change around the sensor may be obtained from thedeviation values of drive currents of the laser and TEC.

Example

One embodiment of a sensor system may be able to operate in amulti-channel mode by using multiplexing technology to monitor thevibration and the temperature variation of the different end windings,simultaneously, as shown in FIG. 36, where N sensors may be employed forsimultaneously monitoring N-point vibrations and the temperaturevariations in the generator. The reflected lights from each sensor maybe detected, 36.1 and the vibrations and temperature variations at eachmonitoring point may be read out and calculated, 36.2, respectively,with the algorithm mentioned above. The detailed process for possiblevibration and temperature variation detections for each sensor isexplained with a flowchart as shown in FIG. 37.

In one embodiment, a sensor system may use a distributed feedback (DFB)laser as the light source and 12 sensors to monitor the vibrations andtemperature variations at six drive end and six non-drive end windingsin the generator as shown in FIG. 39. To measure temperature changeswhile the vibration is measured, the laser drive current may be adjustedone step lower than 1 mA, for example, so that the intensity of thelaser output is not affected excessively. In another example, the systemmay calculate the temperature change as an indirect temperaturemeasurement around the sensor by multiplying the total changed currentvalues with the following parameters: for each ° C. change in laser chiptemperature, the central wavelength of the laser may move by 100picometer; and for the driving current change of every 1 mA, the centralwavelength of the laser may shift by 10 picometer. This controlmechanism may balance the sensor wavelength shift in a change rate up to10 picometer per ° C. The measured data may be transmitted from the dataacquisition system, 38.2, to a computer, 38.3, via a USB cable. In thecomputer, a graphical user interface program may show the detectedmulti-channel vibration waveforms, signal spectrums and temperaturechange trends. Using the user interface program, the user could set thealarm threshold for each monitoring point in the end windings.

Fiber Optic Lead Box Vibration/Temperature Sensors

The vibration and temperature of the lead box in a generator are twoimportant parameters which may be real-time monitored through certaindetection methods. An increase in the vibration amplitude of the leadbox may indicate that the lead box structure has lost its integration,and may become free to move. As a result, excess vibration may causestructure cracking and may result in hydrogen leakage which may increasethe risk of fire. Accompanying this process, there may be excess heatgenerated which may raise the temperatures at different positions of thelead box. An effective detection method may be to use fiber sensortechnology to measure the changes of the optical signal parameters, suchas optical intensity, polarization state or optical phase, induced bythese effects. In this embodiment, a packaged fiber opticvibration/temperature sensor may be used to simultaneously monitor thevibration of the lead box and the temperature changes around the sensor.The movements of the detected fringe signal in high speed and in lowspeed may be used as two characteristics to determine the vibrationstate of the lead box and the temperature variations, respectively.

Description of Technology

As shown in FIG. 31, a vibration/temperature sensor may comprise atwin-grating fiber sensor, 31.1, as a sensing element, a fiberglass reedforming a vibration board, 31.2, and a nylon box with fiberglassfilling, 31.3. The sensing element may be embedded in the fiberglassreed which may be mounted on a pedestal, 31.4, in the sensor box forminga mechanical amplifier.

The sensor's cavity may be formed by two identical fiber gratings (twingratings), 31.5, in respect to the Bragg wavelength and the reflectionrate. In the manufacturing process, the twin gratings may be inscribedin the fiber within a single exposure session by the use of amplitudemasks with two equal slits separated by a designed distance. Thisprocess theoretically guarantees two FBGs be formed with identicaloptical characteristics. The sensor finally may be packaged in a sealedbox to prevent the ingress of dust and oil.

As illustrated schematically by FIG. 32, when the sensor is mounted onan object with mechanical vibration, the reed may vibrate and deform,32.1 periodically owing to an acceleration applied on the reed, 32.2,which may generate a time-varied stress on the optical cavity structureof the sensor and which may, in turn, induce a phase periodic change,regarded as fringe movement, 32.3, in the detected fringe signal.

FIG. 33 is a schematic for the vibration measurement mechanism whichinvolves an algorithm that tunes the laser central wavelength to themid-point of a fringe signal through adjusting the driving current ofthe laser indicated with a letter M, 33.1. This may be regarded as anintersection point or a working point generally used for phasemeasurements in the sensor technology. When the fringe signal moves toleft and right, 33.2, periodically as a result of the vibration of thereed, the working point of the detection system may change, 33.3, alongwith the slope of the target fringe, moving to the upper portion (Hpoint) and the lower portion (L point) periodically, which may give adifferent intensity of the reflected light. This may form an effectivetranslation mechanism to convert a wavelength-changed signal to anintensity-changed signal. In this way, the intensity changes in thedetection signal may then be interpreted as a detected vibration signalin the analog form.

Detection of temperature changes during vibration measurements may becompleted by adjusting the laser central wavelength to track the targetfringe movement. As shown in FIG. 34, when the temperature around thesensor changes, for example from T₁, to T₂, the wavelength of the fringeto be detected may shift accordingly from λ_(C) to λ_(D). This processmay induce the fringe to move toward the long wavelength which, as aresult of working point changing, may induce a decrease of the outputamplitude of the vibration signal. In order to keep the output amplitudeof the vibration signal constant, the laser central wavelength may betuned to dynamically track the target fringe movement. As illustrated inFIG. 34, the laser central wavelength may be adjusted from λ_(P) toλ_(Q), which, theoretically, makes the working point recover to theoriginal position and consequently keeps the vibration signal amplitudeconstant. This fringe-tracking process may be completed by adjusting thelaser drive current based on an error signal measured from the deviationof the vibration signal amplitude.

While adjusting the drive current of the laser diode may be fast enoughto tune the laser central wavelength to track the target fringe movementto readjust the working point, the technique may have a limited tuningrange. Beyond this range, the laser central wavelength may be tuned withthe help of the thermoelectric cooler (TEC) drive current adjustment. Asknown, the laser central wavelength changes linearly with the operationtemperature that may be controlled by the TEC and may have a largetuning span and change rate per ° C. Therefore, besides the laser drivecurrent control, the TEC's drive current control may be used to tune thelaser central wavelength to track the target fringe movement within alarge temperature span. Basically, the laser's drive current control maybe employed for fine wavelength tuning, while the TEC's drive currentcontrol may be employed for wavelength tuning in a larger scale. The twocontrol methods may be alternately employed in the sensor system forachieving an accurate adjustment of the laser center wavelength to trackthe target fringe movement. Based on this control algorithm, thevariations of the vibration signal amplitude, from the working point,movement due to the temperature changes may be compensated. At sametime, the deviation values in TEC's drive current and the laser drivecurrent may be read out as a measured value for estimating thetemperature changes affecting the sensor head. The relationships betweenthe sensor temperature, which results in sensor wavelength shift, maythen be compensated by the laser wavelength shift due to changes in thedrive currents of the laser and the TEC, as shown in FIGS. 35(1), (2)and (3), respectively. From these relationships, it will be appreciatedthat the temperature change around the sensor may be obtained from thedeviation values of drive currents of the laser and TEC.

Example

Embodiments of a sensor system may be able to operate in a multi-channelmode by using multiplexing technology to monitor the vibration and thetemperature variation at six points on the lead box simultaneously, asshown in FIG. 36, where N sensors may be employed for simultaneouslymonitoring N-point vibrations and the temperature variations in thegenerator. The reflected lights from each sensor may be detected, 36.1and the vibrations and temperature variations at each monitoring pointmay be read out and calculated, 36.2, respectively, with the algorithmmentioned above. The detailed process for possible vibration andtemperature variation detections for each sensor is explained with aflowchart as shown in FIG. 37.

In one embodiment, as shown in FIG. 38, a sensor system may use adistributed feedback (DFB) laser as the light source and six sensors,38.4, to monitor the vibrations and temperature variations at sixdifferent points on the lead box in the generator. For example, tomeasure temperature changes while the vibration is measured, the laserdrive current may be adjusted one step lower than 1 mA, so that theintensity of the laser output is not affected excessively. In anotherexample, the system may calculate the temperature change as an indirecttemperature measurement around the sensor by multiplying the totalchanged current values with the following parameters: for each ° C.change in laser chip temperature, the central wavelength of the lasermay move by 100 picometer; and for the driving current change of every 1mA, the central wavelength of the laser may shift by 10 picometer. Thiscontrol mechanism may balance the sensor wavelength shift in a changerate up to 10 picometer per ° C. The measured data may be transmittedfrom the data acquisition system, 38.2, to a computer, 38.3, via a USBcable. In the computer, a graphical user interface program may show thedetected multi-channel vibration waveforms, signal spectrums andtemperature change trends. Using the user interface program, the usercould set the alarm threshold for each monitoring point in the lead box.

Fiber Optic all Flow/Temperature Sensor

The rate and temperature of flow, including by way of example air, gas,water and hydrogen, in a power generator system are two parameters whichmay be real-time monitored during the operation of the power generatorsystem. The majority of transducers used in flow measurement, basically,are the electric-driving type of sensor such as piezoelectric orcapacitance-type sensors. These types of sensors cannot be employed insome special situations such as in the power generator system wherethere exists the large current and high intensity electromagnetic field.With fiber optic sensing technology, it may be possible to let the fiberflow sensors work in a harsh environment and provide them with immunityto electromagnetic interference. The fiber flow sensor embodied here maycomprise a twin-grating sensor as a vortex sensor that may detect thepressure oscillation or vortex-induced vibration in the pipe. The phasevariation in the optical detection signal may be used as acharacteristic to determine the vibration frequency of the vortex field,which may have a direct relationship with the water flow rate to bemeasured.

Description of Technology

The structure of a fiber optic flow sensor assembly is shown in FIG. 40,in which a fiber sensing element as a flow sensor or vortex sensor,40.1, may be installed behind a rectangular metal block called the bluffbody or the vortex generator, 40.2, in a pipe with a diameter D. Thefiber flow sensor may comprise a packaged twin-grating sensor as shownin FIG. 41, in which a stainless steel tube package may be used forwater flow measurement as shown in FIG. 41.1 and a sandwich package withpolyamide films may be used for measurements of air, gas and hydrogenflows, as shown in FIG. 41.2. One embodiment of a fiber flow sensorassembly is schematically shown in FIG. 42.

In operation principle for flow measurement, as illustrated in FIG. 43,when the flow, 43.1, enters the pipe, the vortex generator, 43.2, maycreate two groups of vortices with reverse directions in the flowdownstream. The vortices may generate alternating low pressure zoneswhich may cause any object in this regime to vibrate. This vortexvibration is one kind of pulsating vibration which may be detected witha fiber flow sensor. The vibration frequency called vortex sheddingfrequency f may be linearly proportional to the average flow velocity Vand the flow rate L as shown in FIG. 44. This relationship may beexpressed as:

$\begin{matrix}{{V = {\frac{d}{S_{t}}f}},} & (4)\end{matrix}$

where S_(t) is the Strouhal number and d is the width of the vortexgenerator. The flow rate Q can be calculated with a formula expressedas:

$\begin{matrix}{{Q = {{AV} = {\frac{Ad}{S_{t}}f}}},} & (5)\end{matrix}$

where A is the cross-sectional area available for the water flow.

The structure of twin-grating sensor is shown in FIG. 10. Both sensorand the fiber may be polyamide coated that may provide the sensor anability to work in a harsh environment with high temperatures of up to250° C. Also just like the normal fiber, the polyamide coated fibersensor may be resistant to high voltages and electromagneticinterference.

When the flow is in large velocity, a plug-in type of flow sensorassembly may be utilised and inserted directly into the pipe for flowmeasurement. In this case, a stronger vortex may be generated around thesensor, which may directly cause the sensor to vibrate.

The vortex-induced vibration may cause a periodic change in cavitylength of the fiber flow sensor, which in turn may generate acorresponding phase vibration of the fringe signal around its initialphase value. The phase vibration in the fringe signal may be detectedand converted into an electrical signal with a fringe trackingtechnology as schematically illustrated in FIG. 6.

The fringe tracking technology may comprise a fringe-moving detectiontechnology. As shown in FIG. 6, the input detected signal with multiplefringes may be first filtered in time domain by a time window, 6.1, witha time width equal to that of the fringe, in order to extract a targetfringe, 6.2, to be real-time tracked afterwards. When the laser sourceis frequency modulated with a saw-tooth periodic signal, the fringeposition in this modulation signal period (a time that the fringeappears in a modulation period) may be determined by the initial phasevalue of the fringe signal, 6.3. In the detection of the vortex-inducedvibration, the vortex field may cause the phase of the fringe signal tochange, which in turn may change the fringe position in the modulationsignal period. Using the fringe tracking technology, the position of atarget fringe may be determined and may be transferred in to a voltageoutput with a phase to voltage converter having a linear curve.

With this detection method, the vortex-induced vibration in flow may bedetected as an electrical signal. Finally the vibration frequency may becalculated from the detected electrical signal and the flow rate may beobtained according to the parameters of a flow sensor assembly by usingEquations (4) and (5).

Temperature measurement, with the fiber flow sensor, may be completed byreading the DC level in the detection signal for flow measurements withfringe tracking technology. As shown in FIG. 46, the output waveformfrom flow measurements may comprise two components; one may comprise anAC signal as the vortex-induced vibration and another may comprise a DCsignal that comes from the fringe movement slowly with the temperaturechanges around the sensor. FIG. 46.1 is an output waveform from flowmeasurements in which the v₁ represents an average DC level when thetemperature is 20° C., and FIG. 46.2 is another output waveform obtainedat 30° C., where there is an average DC level v₂, having v₂>v₁. It willbe appreciated that the DC level in the detection signal linearlyincreases with the temperature rising. FIG. 46.3 is a graph describing arelationship between the average DC level in the detection signal andthe temperature around the fiber flow sensor.

A structure of data acquisition system for flow measurement is shown inFIG. 47. The system uses a distributed feedback (DFB) laser source,47.1, with a working wavelength matched with that of the flow sensor.The system may work in a space division multiplexing mode to manage Nfiber flow sensors to form a sensor network, 47.2. Each sensor may beinterrogated by an optical frequency swept probe light from a 1×Noptical power splitter, 47.3, and the returning signal light from thesensor may be detected in the corresponding sensor channel. The signalsfrom all sensor channels may be preliminarily processed in the signalprocessing unit, in order to obtain the vibration waveforms of each flowsensor. These vibration waveforms may then be sent to the computer via aUSB cable. In the computer site, with a user interface program, thefrequency of vortex-induced vibration in accordance with the flowvelocity in each sensor may be measured and finally the flow rate may becalculated.

A flowchart of a user interface program for flow measurements and datadisplay used in the computer site is shown in FIG. 48. With this userinterface program, the user may monitor the current flow rate andtemperature of each fiber flow sensor and may view the change trendcurves as well as set the alarms to corresponding flow sensors.

Example

One embodiment of a flow sensor system is schematically illustrated inFIG. 49, in which a flow sensor assembly, 49.1, may be incorporated inthe water pipe, 49.2 in a power generator system. This fiber flow sensorsystem may simultaneously measure a low rate of water flow below 1 L/minand the water temperature in the pipe from 10˜100° C., respectively. Thedetected signal of the vortex-induced vibration may be transmitted to acomputer through a USB connection, where the flow rate and temperaturemay be calculated and displayed. Another embodiment of a sensor systemthat may be used for simultaneously measuring a large flow rate of water(larger than 10 L/min) and the water temperature in the pipe isschematically illustrated in FIG. 50. In this system, a plug-in typefiber flow sensor may be employed.

Fiber Optic Sensor to Monitor Moisture and Oil Temperature

Moisture in the oil and oil temperature in a generator are twoparameters which may be real-time monitored. An effective detectionmethod may be to use fiber sensor technology to measure the changes ofthe optical signal parameters such as optical intensity, polarizationstate, or optical phase which are related to moisture and temperature.In this embodiment, a multiple-layer polyamide coated, packaged fiberoptic sensor (twin-grating fiber sensor) may be used as a fiber moisturesensor to monitor the moisture in the oil or oil temperature. The phasechanges in the optical detection signal may be used as thecharacteristics to determine moisture in the oil or oil temperature. Asan application, the fiber moisture sensor may be used as a bearing oilsensor for monitoring oil conditions in the generator.

Description of Proposed Technology

The structure of a fiber moisture sensor is shown in FIG. 51.1. Thesensor may comprise a fiber sensing element and a package case. Thefiber sensing element may comprise a multiple-layer polyamide coated,twin-grating fiber sensor with a designed wavelength. For moisturedetection, as shown in FIG. 51.1, the sensor may be packaged inside ananti-oil plastic case as package A that may have many small holes to letthe moisture permeate into the case and arrive at the sensor easily. Foroil temperature detection, as shown in FIG. 51.3, the sensor may besealed inside an anti-oil plastic case as package B.

In operation principle, for moisture measurement, when the twin-gratingfiber sensor is coated with multiple-layer polyamide films on the cavityarea, the polyamide coating may generate an additive strain applied onthe sensor cavity after an annealing treatment. This additive strain maycause the cavity of the sensor to shrink. When the polyamide-coatedtwin-grating fiber sensor is placed in an environment with moisture, thepolyamide coating layers on the fiber may absorb the moisture in theenvironment and start to release this additive strain imposed on thesensor cavity, which may bring about some changes in optical propertiesof the fiber cavity, such as the cavity length increasing. This in turnmay result in a movement of the detected signal phase, or as fringemovement. The amount of the phase variation of the fringe signal may beproportional to the moisture contents, or relative humidity, absorbed bythe sensor.

In respect to signal detection, a movement of the fringe signal inaccordance with the moisture content in time domain can be observed. Therelationship between the moisture content or relative humidity (RH) inthe oil and the phase shift of the fringe signal is schematically shownin FIG. 52. Therefore using the phase measurement technology, one mayobtain the moisture contents absorbed by the sensor.

The phase measurement technology used for moisture detection isschematically explained in FIG. 6 where the detected signal withmultiple fringes may be first filtered in the time domain by a timingwindow, 6.1, with window width being equal to the width of the fringe,6.2; this is to track the fringe as it moves. The tracking may be doneby the distributed feedback (DFB) laser which may be frequency modulatedwith a saw-tooth periodic signal. The fringe position in this modulationsignal period (a time that the fringe appears in a modulation period)may be determined by the initial phase value of the fringe signal, 6.3.With this phase tracking algorithm, changes in the sensor cavity may bedetected, and the phase shift in the detected signal, resulting from themoisture absorbed by the polyamide layer, may be converted into avoltage with an amount corresponding to the moisture content.

For temperature measurement, the detection principle may be the same asthat for moisture measurement. The temperature change also may alter thesensor cavity length and result in a movement of the fringe signal. Thepotential relationship between the temperature and the phase shift inthe fringe signal is shown in FIG. 53.

In one embodiment, a package B type sensor may be employed, which may besealed in an anti-oil plastic case, so the sensor cavity change may onlyresult from the temperature change around the sensor. In respect tosensor interrogation, the data acquisition system may interrogate themoisture sensor and also interrogate the temperature sensor. Onedifference may be that the conversion coefficients and the calibrationsmay be different to these two types of sensors. Since the moisturesensor may have a temperature property, in one embodiment, the detectionresults from the moisture measurement may be compensated with a measuredvalue from the temperature sensor.

A structure of data acquisition system for measurements of the moistureand temperature is shown in FIG. 54. The system uses a distributedfeedback (DFB laser source, 54.1, with a working wavelength matched withthat of the moisture sensor or temperature sensor. The system may workin a space division multiplexing mode to manage N fiber flow sensors toform a sensor network, 54.2. Each sensor may be interrogated by anoptical frequency swept probe light from a 1×N optical power splitter,54.3, and the returning signal light from the sensor may be detected inthe corresponding sensor channel. Finally, the detected voltage signalsfrom different fiber sensors, after preliminarily processing, may betransmitted in a digital form to a computer via a USB cable. In thecomputer site, with a user interface program, the moisture content inthe oil or the oil temperature in each sensor may be calculated.

Reference throughout the specification to “various embodiments,” “someembodiments,” “one embodiment,” “an embodiment,” and the like means thata particular feature, structure, or characteristic described inconnection with the embodiment is included in at least one embodiment.Thus, appearances of the phrases “in various embodiments,” “in someembodiments,” “in one embodiment,” “in an embodiment,” and the like inplaces throughout the specification are not necessarily all referring tothe same embodiment. Furthermore, the particular features, structures,or characteristics may be combined in any suitable manner in one or moreembodiments. Thus, the particular features, structures, orcharacteristics illustrated or described in connection with oneembodiment may be combined, in whole or in part, with the featuresstructures, or characteristics of one or more other embodiments withoutlimitation.

The examples presented herein are intended to illustrate potential andspecific implementations of the embodiments. It can be appreciated thatthe examples are intended primarily for purposes of illustration forthose skilled in the art. No particular aspect or aspects of theexamples is/are intended to limit the scope of the describedembodiments. The figures and descriptions of the embodiments have beensimplified to illustrate elements that are relevant for a clearunderstanding of the embodiments, while eliminating, for purposes ofclarity, other elements.

While various embodiments have been described herein, it should beapparent that various modifications, alterations, and adaptations tothose embodiments may occur to persons skilled in the art withattainment of at least some of the advantages. The disclosed embodimentsare therefore intended to include all such modifications, alterations,and adaptations without departing from the scope of the embodiments asset forth herein.

1. A temperature sensor, comprising: an optical fiber; at least onetwin-grating structure formed on the optical fiber, each twin-gratingstructure comprising: a first optical grating structure, a secondoptical grating structure adjacent the first optical grating structure,and a sensing cavity disposed between the first and second opticalgrating structures, each twin-grating structure selectively responsiveto a unique wavelength of light to generate an optical interferencefringe signal; wherein, for each twin-grating structure, an opticalproperty of the twin-grating structure and a phase of the opticalinterference fringe signal generated by the twin-grating structure aredetermined by a temperature of the twin-grating structure.
 2. The sensorof claim 1, comprising a plurality of twin-grating structures formed onthe optical fiber.
 3. The sensor of claim 1, comprising a polymercoating disposed over each twin-grating structure.
 4. The sensor ofclaim 3, wherein the polymer coating comprises a polyamide coating. 5.The sensor of claim 1, comprising a tube to contain the optical fiberand to reduce non-temperature induced strain applied to the opticalfiber.
 6. A system, comprising: at least one temperature sensor, eachtemperature sensor comprising: an optical fiber; at least onetwin-grating structure formed on the optical fiber, each twin-gratingstructure comprising: a first optical grating structure, a secondoptical grating structure adjacent the first optical grating structure,and a sensing cavity disposed between the first and second opticalgrating structures, each twin-grating structure selectively responsiveto a unique wavelength of light to generate an optical interferencefringe signal; wherein, for each twin-grating structure, an opticalproperty of the twin-grating structure and a phase of the opticalinterference fringe signal generated by the twin-grating structure aredetermined by a temperature of the twin-grating structure; a lasersource coupled to each temperature sensor to input light into theoptical fiber, the laser source configured to output the uniquewavelength of each twin-grating structure formed on the optical fiber;for each temperature sensor, an optical detector coupled to the opticalfiber, wherein when the laser source outputs the unique wavelength of atwin-grating structure formed on the optical fiber, the optical detectorreceives the optical interference fringe signal to generate acorresponding electrical interference fringe signal; and a processorcoupled to the optical detector of each temperature sensor, theprocessor programmed to, for each twin-grating-structure of eachtemperature sensor, extract a target fringe of the optical interferencefringe signal based on a digital representation of the electricalinterference fringe signal and track variation in a phase of the targetfringe over time to determine variation in the temperature.
 7. Thesystem of claim 6, comprising a plurality of temperature sensors.
 8. Thesystem of claim 6, comprising at least one temperature sensor having aplurality of twin-grating structures formed on the optical fiber.
 9. Thesensor of claim 6, wherein at least one temperature sensor comprises apolymer coating disposed over the optical fiber.
 10. The sensor of claim9, wherein the polymer coating comprises a polyamide coating.
 11. Thesensor of claim 6, wherein at least one temperature sensor comprises atube to contain the optical fiber and to reduce non-temperature inducedstrain applied to the optical fiber.
 12. The system of claim 6, whereinthe laser source comprises a tunable laser diode.
 13. The system ofclaim 12, wherein the processor is programmed to control the laser diodeto interrogate a temperature sensor by causing the laser diode to sweepover a working wavelength of the laser diode, the working wavelengthcomprising the unique wavelength of each twin-grating structure formedon the optical fiber of the temperature sensor.
 14. The system of claim13, wherein the processor is programmed to control the laser source in awavelength division multiplexing (WDM) mode.
 15. The system of claim 13,wherein the processor is programmed to control the laser source in afrequency division multiplexing (FDM) mode.
 16. The system of claim 6,wherein a first temperature sensor is disposed in a stator core of agenerator, and wherein the processor is programmed to monitortemperature variations of each twin-grating structure based on thetracked variation in the phases of the corresponding target fringes. 17.The system of claim 6, wherein a first temperature sensor is in contactwith at least one parallel ring connection of a generator, and whereinthe processor is programmed to monitor temperature variations of eachtwin-grating structure based on the tracked variation in the phases ofthe corresponding target fringes.
 18. The system of claim 6, wherein afirst temperature sensor is in contact with at least phase connection ofa generator, and wherein the processor is programmed to monitortemperature variations of each twin-grating structure based on thetracked variation in the phases of the corresponding target fringes. 19.The system of claim 6, wherein a first temperature sensor is in contactwith at least one stator coil connection of a generator, and wherein theprocessor is programmed to monitor temperature variations of eachtwin-grating structure based on the tracked variation in the phases ofthe corresponding target fringes.
 20. The system of 6, wherein a firsttemperature sensor is in contact with at least one main lead connectionof a generator, and wherein the processor is programmed to monitortemperature variations of each twin-grating structure based on thetracked variation in the phases of the corresponding target fringes. 21.The system of 6, wherein a first temperature sensor is in contact withat least one end winding of a generator, and wherein the processor isprogrammed to monitor temperature variations of each twin-gratingstructure based on the tracked variation in the phases of thecorresponding target fringes.
 22. The system of 6, wherein a firsttemperature sensor is in contact with at least one lead box of agenerator, and wherein the processor is programmed to monitortemperature variations of each twin-grating structure based on thetracked variation in the phases of the corresponding target fringes. 23.The system of claim 6, wherein a first temperature sensor comprises onetwin-grating structure and is disposed in a fluid flow of a powergenerator system, and wherein the processor is programmed to monitortemperature variations of the twin-grating structure based on thetracked variation in the phase of the corresponding target fringe. 24.The system of claim 6, wherein a first temperature sensor comprises onetwin-grating structure and is disposed in an oil reservoir of a powergenerator system, and wherein the processor is programmed to monitortemperature variations of the twin-grating structure based on thetracked variation in the phase of the corresponding target fringe.